I believe I have found a solution to the problem mentioned by cewyattjr. It seems the wpDirAuth_auth function never sets the $strFilterQuery variable if the “Authentication Groups” configuration field is empty. This variable is used as the LDAP search filter by the wpDirAuth_retrieveUserDetails command. Adding the following else statement at line 480 appears to have fixed the bug for us.
if($boolUseGroups == 1){
// UseGroups code
} else {
$strFilterQuery = $filterQuery;
}